linux-kernel - Re: [PATCH v6 2/2] x86/refcount: Implement fast refcount overflow protection

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Message-ID: <20170719230129.w52tkcmv5ys7zqsk@treble>
Date:   Wed, 19 Jul 2017 18:01:29 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v6 2/2] x86/refcount: Implement fast refcount overflow
 protection

On Wed, Jul 19, 2017 at 03:50:14PM -0700, Kees Cook wrote:
> >> >  })
> >> > +
> >> > +#define ASM_UNREACHABLE                                                        \
> >> > +       "999: .pushsection .discard.unreachable\n\t"                    \
> >> > +       ".long 999b - .\n\t"                                            \
> >> > +       ".popsection\n\t"
> >>
> >> Just so I understand, we'll get a single byte added for each exception
> >> case, but it'll get discarded during final link?
> >
> > I think it's four bytes actually, but yeah, the section gets stripped at
> > vmlinux link time.
> 
> Right, yes.
> 
> BTW, I think this needs compiler.h coverage instead of the #else in
> compiler-gcc.h (since it's different from how annotate_unreachable is
> used only in compiler-gcc.h. I'll adjust.

Ah, right.  Sounds good.

> Also, in looking at CONFIG_STACK_VALIDATION, do you want it to just
> warn and skip, or do you want to error out the build if validation
> isn't available but it's in the .config?

I think the current warn and skip behavior is fine.  It's usually not a
life-or-death matter, and if it is, you'll be checking the warnings
anyway.

-- 
Josh